| Over the last few years, our group has been at the forefront in the field of photoredox catalysis methodology. These developments have included been the expansion of photoredox cycloadditions and the discovery of new mechanistic pathways for the quenching of the photoexcited state of transition metal catalysts. The first study described herein is the use of electroauxiliaries in photocatalytic [2+2] cycloadditions. Imidazolyl enones are shown to be effective at undergoing reductive cycloadditions to generate cyclobutanes that are readily functionalized. Using this technique we were able to access tri- and tetra-substituted cyclobutanes that were impossible to form under direct photoredox catalysis. There are few examples of the use of electroauxiliaries in photoredox catalysis, and none that describe the activation of substrates towards reduction. This strategy offers insight into the use of electrocatalysts for facilitating the oxidation and reduction of substrates in related synthetic procedures. In a second series of studies, the photoredox radical thiol-ene reaction is investigated. We first describe the efficient coupling between a series of thiols and olefins under mild conditions. Through this study, we have demonstrated the first example of quenching between thiols and photoexcited ruthenium polypyridyl complexes. While this reaction was thermodynamically favorable, we observed rather slow reactivity, indicating that there were significant kinetic issues needing to be addressed. Through attempts to accelerate the reaction, we discovered a novel mechanistic pathway that involved the use of arylamine additives. Further study of our system led us to conclude that the additives were acting as redox mediators via a novel mechanistic pathway. We applied this method inspired towards the thiol-ene reaction of bioorthogonal substrates under mild aqueous conditions. Examples of mediated photoredox processes are still rare, and our successful implementation of this technology demonstrates its future potential in related processes. |
